1. Field of the Invention
The present invention relates to an EL (electroluminescence) display device formed by a semiconductor element (an element using a semiconductor thin film) made on a substrate, and to an electronic apparatus having the EL display device as a display (display portion).
2. Description of the Related Art
Techniques of forming a TFT on a substrate have been greatly advancing in recent years, and development of applications to an active matrix type display device have been progressing. In particular, a TFT using a polysilicon film has a higher electric field effect mobility (also referred to as mobility) than a TFT which uses a conventional amorphous silicon film, and high speed operation is therefore possible.
Shown in FIG. 3 is a general pixel structure of an active matrix type EL display device. Reference numeral 301 in FIG. 3 denotes a TFT which functions as a switching element (hereafter referred to as a switching TFT), reference numeral 302 denotes a TFT which functions as an element (hereafter referred to as an electric current control element) for controlling electric current provided to an EL element 303, and 304 denotes a capacitor (storage capacitor). The switching TFT 301 is connected to a gate wiring 305 and to a source wiring (data wiring) 306. A drain of the electric current control TFT 302 is connected to the EL element 303, and a source of the electric current control TFT 302 is connected to an electric current supply wiring 307.
A gate of the switching TFT 301 opens when the gate wiring 305 is selected, a data signal of the source wiring 306 is stored in the capacitor 304, and a gate of the electric current control TFT 302 opens. After the gate of the switching TFT 301 closes, the gate of the electric current control TFT 302 remains open in accordance with the electric charges stored in the capacitor 304, and the EL element 303 emits light during that period. The amount of light emitted by the EL element 303 is changed by the amount of electric current.
In other words, the amount of electric current flowing in the electric current control TFT 302 is controlled by the data signal input from the source wiring 306 in an analog drive gradation display, and the amount of light emitted by the EL element thereby changes.
FIG. 4A is a graph showing the transistor characteristics of the electric current control TFT 302, and reference numeral 401 denotes an Id-Vg characteristic (also referred to as an Id-Vg curve). Id is a drain current, and Vg is a gate voltage here. The amount of electric current flowing with respect to an arbitrary gate voltage can be found with this graph.
A region of the Id-Vg characteristic shown by a dotted line 402 is normally used in driving the EL elements. An enlargement of the region enclosed by the region 402 is shown in FIG. 4B.
The shaded region in FIG. 4B is referred to as a subthreshold region. In practice, this indicates a region having a gate voltage in the neighborhood of the threshold voltage (Vth) or below, and the drain current changes exponentially with respect to changes in the gate voltage within this region. Electric current control is performed in accordance with the gate voltage by using this region.
The data signal input to the pixel when the switching TFT 301 in FIG. 3 is open is first stored in the capacitor 304, and then the signal becomes the gate voltage of the electric current control TFT 302, as is. The drain current is determined at this time by a one to one correspondence with respect to the gate voltage, in accordance with the Id-Vg characteristic shown in FIG. 4A. Namely, a predetermined electric current flows in the EL element 303 in correspondence with the data signal, and the EL element 303 emits light with the amount of light corresponding to the amount of current flow.
The amount of light emitted by the EL element is thus controlled by the input signal, and gradation display is performed by controlling the amount of light emitted. This method is referred to as analog gradation, and gradation display is performed by changing the amplitude of the signal.
However, the above analog gradation method has a disadvantage of being extremely weak with respect to dispersions in the TFT characteristics. For example, suppose that the Id-Vg characteristic is a switching TFT and differs from that of a switching TFT of an adjacent pixel displaying the same gradation (a case of an overall positive of negative shift).
In this case the drain current of each switching TFT differs on the order of the dispersion, and the gate voltages applied to the current control TFTs of each pixel therefore also differ. In other words, the electric current flowing differs for each of the EL elements, and as a result, the amount of light emitted also differs, and the same gradation display cannot be performed.
Further, even supposing that equal gate voltages are applied to the electric current control TFTs of each pixel, the same drain current cannot be output if there are variations in the Id-Vg characteristics of the electric current control TFTs. In addition, even if equal gate voltages are applied, the amount of electric current output differs greatly if even small deviations exist in the Id-Vg characteristics when using a region in which the drain current changes exponentially with respect to changes in the gate voltage, as is clear from FIG. 4A. The amount of light emitted by adjacent pixels will differ greatly as a result.
In practice, there is a multiplier effect between dispersions in both the switching TFTs and the electric current control TFTs, and this makes achieving the conditions more difficult. Thus the analog gradation method is extremely sensitive with respect to variations in the TFT characteristics, and this becomes an obstacle to multiple colorization of a conventional active matrix EL display device.